This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 99-48598 filed in Korea on Nov. 4, 1999; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a spot size converter which is an optical transceiver device used as an important part of the field related to optical communications, and more particularly, to a double core spot size converter, in which a double core for spot size conversion of an optical beam is fabricated by a selective area growth method, and a method for fabricating the double core spot size converter.
2. Description of the Related Art
In optical communications, a spot size converter is usually used for converting the spot size of an optical beam which is emitted from an optical device to a beam size within the optical device and applying the spot size converted optical beam to an optical fiber so that the optical beam may have the same size within the optical device, which is the source of an optical signal, and the optical fiber, which is an optical signal transmitting medium. The spot size converter is usually applied to a laser diode, a semiconductor optical amplifier, a modulator, a photodetector or a wavelength converter.
To reach the optical communication age earlier, it is essential to reduce the prices of parts related to the optical communication. To reduce the price of parts related to optical transmission and reception, an optical device which does not use a lens has been fabricated in the form of a package. To fabricate optical devices having high optical coupling efficiency without a lens, beam sizes in an optical device and an optical fiber must be similar to each other. However, generally, the beam is a very large size, about 9 xcexcm, in an optical fiber while the beam size is about 1 xcexcm in a semiconductor optical device. Because of the large difference between the beam sizes in the optical device and the optical fiber, optical coupling efficiency is quite poor. To overcome the problem, a method of coupling a spot size converter (SSC) for enlarging the mode size at the end of a device has been developed.
FIG. 1 is a partially sectioned perspective view for showing the structure of a conventional double core SSC. As shown in FIG. 1, an active waveguide 10 has a negative taper in a lateral or vertical direction and a passive waveguide 20 has a positive taper so that the mode which is locked to the active waveguide 10 can be adiabatically passed to the passive waveguide 20. In this structure, the beam size of a device can be adjusted using a passive waveguide only.
In other words, according to the conventional technology shown in FIG. 1, etching is performed in the lateral direction to give a negative taper to the active waveguide 10 for the purpose of reducing the confines of the active waveguide 10. To implement this method, lateral etching is essential and accurate control of the etching rate is also required because only the passive waveguide 20 must exist in a SSC region. That is, a conventional SSC is fabricated such that wet or dry etching is used in forming a waveguide to achieve a small optical confinement factor and to thus increase the beam size.
Conventional SCC fabrication methods can be classified into four groups, as shown in Table 1. Advantages and disadvantages of each method are arranged in Table 1. Other than a selective area growth (SAG) method, it is essential to adjust the width of a waveguide to 0.2 xcexcm or less by performing accurate etching. On the other hand, according to the SAG method, crystallinity of an active layer is sacrificed for a narrow beam divergence angle and an accurate design and fabrication process of a waveguide are required to obtain a circular beam. In a butt-joint method, it is very difficult to ensure a crystal growth condition in which smooth transition of a mode can be achieved between a SSC region and an active region. Thus, such method requires much time and study. In the case of a double core structure, an active waveguide is tapered in a lateral or vertical direction to increase the mode size so that mode in the active waveguide can be coupled to an underlying passive waveguide, thereby adjusting the beam divergence angle. Because the refractive index, thickness and width of the passive waveguide can be adjusted regardless of the active waveguide, the beam size can be easily adjusted. Since this method employing the double core structure requires stable adjustment of etching Width and depth, it is essential to ensure a dry etching process. However, it is very difficult to obtain a smooth profile in a tapered region when using the dry etching process. In the case of adjusting the width of a waveguide by using an etching method to adjust the beam size, it happens that the end of the waveguide having a very narrow width is collapsed at a high temperature by a mass transport phenomenon during a regrowth for making a buried heterostructure, thereby warping the shape of a beam.
To solve the above problems, it is an objective of the present invention to provide a double core spot size converter and a fabrication method thereof, for improving the optical coupling efficiency between an optical device and an optical fiber by reducing astigmatism and the far field angle of a beam emitted from the optical device.
Accordingly, to achieve the above objective, there is provided a double core spot size converter including a lower clad layer, a lower passive waveguide which is formed on the lower clad layer to a predetermined width and thickness and determines the pattern of an emitted beam, a spacer which is formed on the lower passive waveguide to a predetermined thickness, an active waveguide which is formed on the spacer to a predetermined thickness, an upper passive waveguide which is formed in the shape of a negative taper connected to the active waveguide without a break on the spacer, for spot size conversion, and an upper clad layer which is formed on the active waveguide and the upper passive waveguide.
In the present invention, the lower passive waveguide is at least 0.3 xcexcm wide and has a thickness of 1 xcexcm or less. The spacer has a thickness of 3 xcexcm or less. In the taper structure of the upper passive waveguide, the increase in thickness has a ratio of at least 1.5. The upper passive waveguide is formed such that its end meeting a beam emitted facet stops in advance of the end of the lower passive waveguide to allow a mode transition from the upper passive waveguide to the lower passive waveguide. The lower clad layer is formed of n-InP. The lower passive waveguide is formed of InGaAsP. The spacer is formed of InP. The active waveguide and the upper passive waveguide are formed of InGaAsP. The upper clad layer is formed of p-InP. The double core spot size converter also includes first and second current blocking layers which are formed of p-InP and n-InP, respectively, on both sides of the waveguides, for insulation.
There is also provided a method for fabricating a double core spot size converter. The method includes the steps of (a) sequentially growing a lower passive waveguide and a spacer on a lower clad layer acting as a substrate, (b) forming a selective area growth pattern on the spacer and simultaneously growing an upper passive waveguide, which is a spot size conversion area in a tapered structure, and an active waveguide, which is an active area, using selective area growth, (c) partially removing a thin portion at the edge of the tapered structure by performing an etching process on the upper passive waveguide to complete the upper passive waveguide, (d) partially etching the portion of the clad layer under the side of a waveguide comprising the upper passive waveguide and the active waveguide and performing a regrowth process on the etched portion to grow first and second current blocking layers, forming an insulation structure, and (e) growing an upper clad layer on an exposed portion of the upper passive waveguide, the active waveguide and the first and second current blocking layers.
In the step (a), the lower passive waveguide is grown to a thickness of 1 xcexcm or less, and the spacer is grown to a thickness of 3 xcexcm or less. In the step (b), the upper passive waveguide is grown by the selective area growth such that the increase in thickness has a ratio of at least 1.5. In the step (c), the upper passive waveguide is etched such that its end meeting a beam emitted facet stops in advance of the end of the lower passive waveguide to allow a mode transition from the upper passive waveguide to the lower passive waveguide. In the step (d), the lower passive waveguide is formed to have a width of at least 0.3 xcexcm. The lower clad layer is formed of n-InP. The lower passive waveguide is formed of InGaAsP. The spacer is formed of InP. The active waveguide and the upper passive waveguide are formed of InGaAsP. The upper clad layer is formed of p-InP. The first current blocking layer is formed of p-InP. The second current blocking layer is formed of n-InP.